L-Sorbose is a useful substance as a raw material for synthesis of vitamin C (L-ascorbic acid) and, generally, it is produced by L-sorbose fermentation wherein D-sorbitol is oxidized by a L-sorbose-producing bacterium a representative example of which is one of bacteria of Acetobacter, Gluconobacter suboxydans (or Acetobacter . suboxydans).
Usually, for mass production, there is employed a so-called batchwise procedure wherein a main fermentor having a large volume is used. Although, recently, a continuous process has been studied, it can scarcely be employed for mass production because of instability of growth of the producing bacterium, contamination by other bacteria, remaining of unreacted sorbitol and the like. In such a batchwise fermentation procedure, for every batch, a seed culture is prepared from isolated pure bacterial cells of the producing bacterium by culturing them through several growth steps wherein a culture grown in a smaller culture tank is transferred in turn to a tank having a larger volume such as from a slant culture to a seed culture through a pre-culture and, finally, the seed culture thus obtained is used for a main fermentation to oxidize D-sorbitol to obtain L-sorbose.
In the above several growth steps, it is necessary to prevent contamination by other bacteria and to employ complicated operation for obtaining a pure culture and, further, much labor and many hours are required.
The present invention has been done in order to improve the above requirements in mass production of L-sorbose by a batchwise fermentation procedure such as complicated operation, much labor and many hours, and to carry out mass production of L-sorbose for a long period of time stably and economically. And, it has been completed based on the present inventors' finding that it is possible to subject a seed culture to subculture without lowering growth and oxidizing abilities of a L-sorbose-producing bacterium and thereby it is possible to eliminate the above preparation of a seed culture by several growth steps.
U.S. Pat. No. 4,156,630 discloses a process of fermentation wherein the microorganisms or enzymes are recycled into the fermentor after separating therefrom the fermentation filtrate and an apparatus therefor. However, it does not teach subculture of seed.